Water may contain many different kinds of contaminants including, for example, particulates, chemicals, and microbiological organisms, such as bacteria, viruses, and protozoa. In a variety of circumstances, these contaminants must be reduced in concentration or completely removed before the water can be used. For example, in many medical applications and in the manufacture of certain electronic components, extremely pure water is required. As a more common example, any harmful contaminants must be removed from the water before it is potable, i.e., fit to consume.
The quality of water varies widely around the world. In the U.S. and other developed countries, drinking water is typically municipally treated. During that treatment, contaminants, such as suspended solids, organic matter, heavy metals, chlorine, bacteria, viruses, and protozoa are removed from the water before it is discharged to the homes of consumers. However, equipment malfunction and/or infrastructure breakdown and other problems with water treatment utilities can lead to incomplete removal of the contaminants.
Many developing countries are without water treatment utilities. As such, there are deadly consequences associated with exposure to contaminated water, as many developing countries have increasing population densities, increasingly scarce water resources, and no water treatment utilities. It is common for sources of drinking water to be in close proximity to human and animal waste, such that microbiological contamination is a major health concern.
As a result of waterborne microbiological contamination, an estimated six million people die each year, half of which are children under 5 years of age. In 1987, the U.S. Environmental Protection Agency (herein “EPA”) introduced the “Guide Standard and Protocol for Testing Microbiological Water Purifiers”. This guide standard and protocol provides guidelines and performance requirements for drinking water treatment systems that are designed to reduce specific health related contaminants in public or private water supplies. The requirements are that the effluent from a water treatment system exhibits 99.99% (or equivalently, 4 log) removal of viruses, 99.9999% (or equivalently, 6 log) removal of bacteria, and 99.9% (or equivalently, 3 log) removal of protozoa (cysts) against a challenge.
Under the EPA guide standard and protocol, in the case of viruses, the influent concentration should be about 1×107 viruses per liter (PFU/L), and in the case of bacteria, the influent concentration should be about 1×108 bacteria per liter (CFU/L). Because of the prevalence of Escherichia coli (E. coli, bacterium) in water supplies, and the risks associated with its consumption, this microorganism is used as the bacterium in the majority of studies. Similarly, the MS-2 bacteriophage (or simply, MS-2 phage) is typically used as the representative microorganism for virus removal because its size and shape (i.e., about 26 nm and icosahedral) are similar to many viruses. Thus, a filter's ability to remove MS-2 bacteriophage demonstrates its ability to remove other viruses.
It was believed by those skilled in the relevant art that small suspended particles, for example, bacteria and viruses, are best filtered by filters having small interstitial spacing between filter particles. Small space between filter particles is best achieved by close packing of the filter particles. One way to achieve close packing is described in published PCT application WO 00/71467 A1, in the name of Tremblay et al., which teaches the use of small particles to fill in the spaces between larger particles. This provides close packing by using filter particles having a bi-modal size distribution. Moreover, U.S. Pat. Nos. 5,922,803 and 6,368,504 B1, issued to Koslow et al. and Kuennen et al., respectively, teach the general principle of using filter particles having a narrow particle size distribution, that is, particles that are generally all the same size, to insure that the interstitial spacing between particles is relatively uniform. The average particle size of these two narrow particle size distribution patents ranges from 80 μm to 45 μm. These patents describe filters that achieve either a relatively high level of virus removal with a high pressure drop across the filter or low virus removal since the average filter particle size is relatively large although the size distribution is narrow.
A high pressure drop across a filter can cause reduced flow and other problems that are viewed negatively by filter users. Going to smaller particle sizes, for example, less than 45 μm was believed to further exacerbate the pressure drop across a filter. Moreover, those skilled in the art will appreciate that the pressure drop has a direct impact on flow rate through a filter block. Consumers typically have water delivered to their homes at a fixed pressure (from the municipality or from a pump in their well, for example). Thus a filter block with a high pressure drop will have a slower flow rate than one with a smaller pressure drop. As can be appreciated, consumers do not like to wait long periods of time for their water, so high flow rates are preferred. As such, filter blocks with low pressure drops are necessarily preferred by consumers. Thus, there exists a need for filters, processes for manufacturing filter materials and filter materials which are capable of removing bacteria and/or viruses from a fluid without the disadvantageous increase in pressure drop exhibited by filters of the prior art.